Cutting inserts for material working, in particular for cutting metal working, comprise a hard metal, cermet or ceramic substrate body which in most cases is provided with a single-layer or multi-layer surface coating to improve the cutting and/or wear properties. The surface coatings comprise mutually superposed hard material layers of carbides, nitrides, oxides, carbonitrides, oxynitrides, oxycarbides, oxycarbonitrides, borides, boronitrides, borocarbides, borocarbonitrides, borooxynitrides, borooxocarbides or borooxocarbonitrides of the elements of groups IVa to VIIa of the periodic system and/or aluminium, mixed metal phases and phase mixtures of the afore-mentioned compounds. Examples of the afore-mentioned compounds are TiN, TiC, TiCN and Al2O3. An example of a mixed metal phase in which in a crystal a metal is partially replaced by another is TiAlN. Coatings of the afore-mentioned kind are applied by CVD methods (chemical vapour phase deposition), PCVD methods (plasma-supported CVD methods) or by PVD methods (physical vapour phase deposition).
Inherent stresses obtain in almost every material as a consequence of mechanical, thermal and/or chemical treatment. In the production of cutting inserts by coating a substrate body by means CVD methods, inherent stresses result for example between the coating and the substrate and between the individual layers of the coating from the different coefficients of thermal expansion of the materials. The inherent stresses can be tension or compression inherent stresses. When a coating is applied by means of PVD methods additional stresses are introduced into the coating by ion bombardment when using that method. In coatings applied by means of PVD methods compression inherent stresses generally prevail whereas CVD methods usually produce tension inherent stresses in the coating.
The effect of the inherent stresses in the coating and in the substrate body can be without a considerable influence on the properties of the cutting insert, but they can also have considerable advantageous or disadvantageous effects on the wear resistance of the cutting insert. Tension inherent stresses which exceed the tensile strength of the respective material cause fractures and cracks in the coating perpendicularly to the direction of the tension inherent stress. In general a certain amount of compression inherent stress in the coating is desired as surface cracks are prevented or closed thereby and the fatigue properties of the coating and thus the cutting insert are improved. Excessively high compression inherent stresses however can lead to adhesion problems and spalling of the coating.
There are 3 kinds of inherent stresses: macrostresses which are almost homogenously distributed over macroscopic regions of the material, microstresses which are homogenous in microscopic regions like for example a grain, and non-homogenous microstresses which are also non-homogenous on a microscopic plane. From a practical point of view and for the mechanical properties of a cutting insert macrostresses are of particular significance.
Inherent stresses are usually specified using the unit Megapascal (MPa), wherein tension inherent stresses have a positive sign (+) and compression inherent stresses have a negative sign (−).
It is known that hard metal cutting tools which are coated with hard material layers like for example TiN, TiC, TiCN, TiAlN, Al2O3 or combinations thereof can have excellent wear resistance but they can rather fail in a situation involving thermomechanical alternating loading in interrupted cutting operations as for example in crankshaft milling, by virtue of a loss in toughness in relation to uncoated cutting tools or those which are coated by means of PVD methods. A similar consideration applies to turning working in an interrupted cutting mode or under disadvantageous cutting conditions (for example vibrations caused by the machine or the workpiece clamping). For such applications under disadvantageous conditions, hitherto CVD coatings with a limited layer thickness (rarely more than 10 μm) are used as the embrittlement of the cutting material, caused inter alia by tensile stresses, increases with the thickness of the CVD coating. Highly wear-resistant kinds of cutting materials in contrast frequently involve layer thicknesses of 20 μm or more, but they can only be used in a continuous cutting mode under advantageous conditions. In the case of cutting inserts for turning working of steel or cast iron therefore both high wear resistance and also high toughness are desired, these being two properties which frequently cannot be achieved at the same time.
DE-A-197 19 195 describes a cutting insert have a multi-layer coating which is deposited in a continuous CVD method at temperatures between 900° C. and 1100° C. The change in material in the multi-layer coating from one layer to the next occurs due to a change in the gas composition in the CVD method. The outermost layer (cover layer) comprises a single-phase or multi-phase layer of carbides, nitrides or carbonitrides of Zr or Hf, in which internal compression inherent stresses prevail. The subjacent layers comprise TiN, TiC or TiCN and without exception have internal tension inherent stresses. The compression inherent stress measured in the outer layer is between −500 and −2500 MPas. That is intended to improve fracture toughness.
To increase the compression inherent stresses in the coating on the substrate body of cutting inserts or other tools it is known for them to be subjected to a mechanical surface treatment. Known mechanical surface methods are brushing and jet blasting treatment. Jet blasting treatment involves directing a fine-grain jet blasting agent of grain sizes of up to about 600 μm by means of compressed air under increased pressure on to the surface of the coating. Such a surface treatment can reduce tension inherent stresses or compression inherent stresses in the outermost layer and also in the subjacent layers. In regard to jet blasting treatment a distinction is drawn between dry jet blasting treatment in which the fine-grain jet blasting agent is used in the dry condition and wet jet blasting treatment in which the granular jet blasting agent is suspended in a liquid.
It was found that the choice of the jet blasting agent has a considerable influence on the changes in the inherent stresses in the coating and in the substrate of the cutting insert, in particular the hardness of the jet blasting agent in relation to the hardness and thickness of the coating. It was possible to show that, when using a jet blasting agent whose hardness is greater than that of the outermost layer of the coating, the wear mechanism in the jet blasting procedure is abrasion and high compression stresses occur only at the near surface regions of the layer to about 1 μm depth of penetration, and they very quickly relax again. In deeper layers or in the substrate there is substantially no reduction in the tension stresses or increase in the compression stresses. The inherent stress prevailing in the substrate after the coating process remains unchanged. It is not possible to achieve an increase in the toughness of the tool.
If the hardness of the jet blasting agent is equal to the hardness of the outermost layer of the coating then the wear mechanism in the jet blasting operation is surface spalling and there are high compression stresses which can act into deeper coating layers and in dependence on the layer thickness also into the substrate. With thick layers (>>10 μm) with wet jet blasting the stress in the substrate can be only little altered and tensile strength can be increased. If nonetheless there is a wish to increase the compression stress in the substrate even with thick layers, it is necessary to use very long dry jet blasting operations, which leads to an increase in lattice dislocations and can cause adhesion problems with the coating.
If the hardness of the jet blasting agent is less than that of the outermost layer of the coating surface bombardment (shot peening) is also substantially assumed as the wear mechanism of that outermost layer. The wear rate at the outermost coating is lower so that longer jet blasting times are possible without any layer removal worth mentioning. A further advantage is that in that respect no or only slight degrees of dislocation are produced in the uppermost layers of the coating. Depending on the respective choice of the method parameters (inter alia jet blasting agent, pressure, duration and angle) and layer thickness inherent stress changes can be achieved in different depths of the composite consisting of the hard metal and the coating. In other words, as a result of the jet blasting treatment, compression stresses can occur in different layers of the coating and also in the substrate.
DE-A-101 23 554 describes a jet blasting method using a granular jet blasting agent of a maximum diameter of 150μ. As a result, a reduction in tension inherent stresses or an increase in compression stresses is achieved in the outermost layer and the subjacent layers, preferably in the region near the surface of the substrate. Preferably compression stresses of some GPa are achieved in the uppermost layers.
Cutting inserts with an outer wear protection layer of alpha or gamma aluminium oxide for metal working have been in use for many years and are described in detail in the literature. It has been found that alpha aluminium oxide coatings with given preferential directions of crystal growth in deposition in the PVD or CVD methods can have particular advantages, in particular an improved wear characteristic, in which respect for different applications of the cutting insert different preferential orientations of the aluminium oxide layer can also be particularly advantageous. The preferential orientation of crystal growth is generally specified in relation to the planes defined by way of the Miller indices, for example the (001) plane, of the crystal lattice and are referred to as texture or fibre texture and are defined by way of a so-called texture coefficient (TC). For example cutting inserts with a wear layer of alpha aluminium oxide with (001) texture have advantages over other preferential orientations in steel machining in respect of relief face wear and crater wear as well as plastic deformation.
US-A-2007/0104945 describes cutting tools with α-Al2O3 wear layers with (001) texture and a columnar microstructure. That preferential orientation is revealed by high intensities of the (006) peak in the X-ray diffraction spectrum (XRD diffractogram) and is achieved by both nucleation and also growth of the α-Al2O3 layer being performed in the CVD method under given conditions. Nucleation is effected at ≦1000° C. on a TiAlCNO bonding layer by a multi-stage method in which the substrates are successively exposed to defined gas concentrations of TiCl4 and AlCl3, flushing steps in N2 and defined H2O concentrations. Nucleation of α-Al2O3 is then continued by growth without catalytic additives and finally at 950 to 1000° C. layer growth to the desired layer thickness takes place under a defined concentration ratio of CO/CO2 and in the presence of typical catalysts like H2S, SO2 or SF6, in concentrations≦1% by volume.
EP 1 953 258 also describes cutting tools with α-Al2O3 wear layers with a (001) texture on hard metal substrates with an edge zone enriched with Co binder. The preferential orientation of the α-Al2O3 wear layer is achieved by nucleation similarly to US-A-2007/0104945, but it will be noted that as a departure therefrom upon further growth of the layer the CO/CO2 ratio gradually increases.
EP-A-2 014 789 also describes cutting tools with α-Al2O3 wear layers with a (001) texture on hard metal substrates with an edge zone enriched with Co binder, which are said to be suitable in particular for cutting machining of steel at high cutting speeds, in particular for steel turning.